The invention can be used where radiation is interrupted periodically by a light stop and the radiation, so modulated, must be switched over for two optical paths. Moreover, control signals are provided for the further processing of measured values. The invention is suitable preferably for infrared spectrophotometers, which work according to the double-beam method.
In the DDR Patent No. 148,985, a device for the modulation, light-path switch-over and the derivation of the necessary control signals is described for the infrared range of the spectrum in connection with a spectrophotometer. A synchronous motor drives a chopper disk, which carries three control tracks. The outer track, on the one hand, serves for interrupting the radiation of the photometer periodically, the radiation being modulated with f.sub.m. On the other hand, the control signal for processing the measured value, for example, for controlling a voltage-frequency convertor, is generated on an optocoupler. Because of the phase shift by the radiation detector and the electronic amplifier, the optocoupler must be adjustable for the phase adjustment, that is, it must be rotatable about the axis of the chopper disk. Two further optocouplers are disposed to be stationary. They supply the pulse and direction of rotation signals for the optical switch-over device. The switch-over device itself comprises two pivoting mirror groups, each pivoting mirror being driven by a stepping motor. The first group of pivoting mirrors is disposed in front of the sample space, the second group of pivoting mirrors behind the sample space. The test sample and the reference sample are placed in the sample space. The radiation is passed either through the test sample or the reference sample, depending on the momentary position of the two groups of pivoting mirrors. The measured value M or the reference value R is thus formed. The two groups of pivoting mirrors must work synchronously with one another and with the chopper disk. For this purpose, each of the two groups of pivoting mirrors carries two optocouplers. A logic circuit compares the information of the optocouplers of the chopper disk with that of the optocouplers of the groups of pivoting mirrors. For synchronizing the device for the modulation and the light-path switch-over, six optocouplers are required; if the optocoupler for producing the control voltage, for example for the voltage-frequency converter, is included, a total of seven optocouplers is required. This means a large expenditure for mechanical parts and electronic components, lines and plug-in connections and the expense of installing the groups and adjusting and checking them. The reliability, which is influenced by the number of parts and components and by the difficulties encountered in installing them, is slight.
It is an object of the invention to provide, at the least possible cost, a device for synchronous modulation, synchronous light-path switch-over and control-signal derivation and to increase the reliability substantially relative to the state of the art. Moreover, the inventive device shall be as universally usable as possible.